Methods for marking thermoplastic resins include the spraying of ink by ink jet processes, screen printing, and ink writing. Problems with heat impact resistance (peeling of the ink surface), weatherability (discoloration of the ink) and wear resistance (wearing of the ink surface) are generally encountered with the use of these methods. In ink jet processes and screen printing, ink is applied to the resin surface and dried, thereby causing it to adhere; attention must be paid to this point in the selection of the ink. In ink embedding processes, either depressions are applied beforehand during molding in the resin marking places or engraving is carried out after molding; marking is then carried out by embedding ink or paint at those places. However, in methods where depressions are applied during molding, the marking shape is limited by the mold, making it difficult to change the marking shape for each product. Hence, in processes where engraving is carried out, a great deal of time is taken up with marking. In addition, because it is necessary that ink or paint be embedded afterward and the excess ink or paint removed, the steps become complicated, making such processes inappropriate for marking mass-produced products.
For the above reasons, much industrial use has been made recently of laser marking methods, which are non-contact, have a rapid marking speed, are automated, and enable easy process control. Laser marking methods are processes that use laser light to place marks, bar codes or images on the surface of metal, ceramic, macromolecular organic materials or the like.
Laser marking involves irradiation of the target surface with laser light. In a first type of laser marking process, marking is effected by utilizing changes in the surface state (toughening or forming depressions) brought about by erosion of the irradiated portion. In a second type of laser marking process, as described in Japanese Published Unexamined Patent Application [Kokai] Nos. 56-144,995 (1981), 60-155,493 (1985), 60-166,488 (1985), 60-172,590 (1985), 60-47,065 (1985), 62-50,360 (1987) and 3-10,884 (1991), marking is carried out by using the decoloration and discoloration of pigments and dyes in the irradiated portion. In a third type of laser marking process, as described in Japanese Examined Patent Application [Kokoku] No. 2-47,314 (1990), marking is carried out by using changes in the surface state (projections) due to foaming of the stock resin. Here, in laser marking process (3) that utilizes a change in state, unpolymerized monomers and decomposition products present in that portion of the resin to which intense heat has been applied by means of laser light are volatilized, causing those places to foam. Thus, marking is carried out to scatter and reflect light, and also to cause the resin to rise.
However, in marking by toughening or forming depressions in the surface (1), the boundary between the areas irradiated with laser light and the areas not irradiated is indistinct. There are cases where different types of resin are molded into a laminar structure and just the resin in the surface layer is removed. However, the production steps in this process become extremely complex, and there are problems over which layer erosion occurs down to. In the laser marking process (2) involving the decoloration or discoloration of pigments and dyes, the type and color of the resin is limited by the type of pigment or dye, which represents a drawback. Moreover, because the pigments and dyes are readily decolored or discolored by heat, the region of discoloration broadens and the contours of letters or the like become indistinct. In the laser marking process (3) that utilizes changes in state due to foaming, the optimal conditions for each resin must be investigated. Moreover, when one considers that a resin is not often used alone, in cases where the surface of a resin composition made of a plurality of ingredients is marked, the appropriate conditions in the various compositions must be carefully examined. Because some resins do not readily foam, there are limits on the resins that can be used.
Hence, the object act of this invention is to provide resin compositions to which distinct and secure markings can be easily applied in a short time.